Abstract: 【Objective】 To study the effects of corn leaves treated with methyl jasmonate(MeJA)on the growth and development of Mythimna separata,reveal the relevant insect resistance mechanism of corn,and provide a theoretical basis for the use of plant resistance for pest control.【Method】 Spraying 100 μmol/L of MeJA solution on corn seedlings. After 24 h,fed the corn seedlings to the first instar larvae of M. separata,continued feeding until pupation,observed and analyzed the growth and development of M. separata. Then,quantitative PCR was used to study the expression changes of key genes in the jasmonic acid pathway after treatment with exogenous MeJA,and the qualitative and quantitative analysis of related hormones in the treated corn leaves by ultra performance liquid chromatography-tandem mass spectrometry(UPLC-MS/MS).【Result】 Compared with the control,after feeding on corn leaves treated with MeJA,the body weight of the 6^th instar M. separata larvae was reduced by 11.67%,the pupal weight was reduced by 18.20%,the emergence rate was reduced by 7.35%,the adult survival time was shortened by 0.93 d,and the pupal period was extended by 0.27 d.The fluorescence quantitative PCR detection results showed that after 24 h of exogenous MeJA treatment,the expression of ZmLox gene was significantly up-regulated(P<0.05,the same below),reaching 5.57 times that of the control;ZmAos gene expression increased significantly after treatment for 12 h,then it decreased to the level of the control after 24 h.The expression of ZmAoc gene and ZmOpr gene did not change significantly at 12 and 24 h after treatment(P>0.05,the same below). UPLC-MS/MS qualitative and quantitative analysis results showed that after spraying MeJA for 24 h,the content of dihydrojasmonic acid(H₂JA)in corn leaves increased significantly,which was 5.67 times that of the control.While there was no significant difference in the content of jasmonic acid(JA),jasmonic acid-isoleucine(JA-ILE)and MeJA.【Conclusion】 Spraying of MeJA induces corn defense ability and affects the growth and development of the M. separata. The induced expression of ZmLox and the production of hormone H₂JA would be an important mechanism of corn resistance to insects.